Back to EveryPatent.com
United States Patent |
5,267,219
|
Woodward
|
November 30, 1993
|
Acoustic range-finding system
Abstract
In an acoustic pulse-echo ranging system, a high impedance piezoelectric
transducer element is connected via a transformer to a transmitter, and
also directly to a high impedance input of a receiver through a network,
in parallel with the transducer element which includes a limiting resistor
having a value which is low compared with the impedance of the transducer
and a pair of oppositely connected diodes connected across the receiver
input, the diodes being in series with the resistor and conducting to
complete a damping circuit for large amplitude signals during
transmission, and to limit the amplitude of signals applied to the
receiver. When the signals are not of sufficient amplitude to cause the
diodes to conduct, the damping circuit is normally rendered ineffective,
but a further circuit is provided in parallel with the diodes to
selectively switch into the network a further smaller resistance which
acts both to form an attenuator at the receiver input in conjunction with
the damping resistor, and to restore the damping effect. By this means,
the dynamic range required by the receiver is much reduced, and effective
damping can be applied selectively during reception to reduce the effect
of ringing at short ranges. A tuned circuit formed with the transducer by
the transformer through which the transmitter drives the transducer is
tuned by means of an air gap in the transformer core, which may be
adjustable.
Inventors:
|
Woodward; Steven J. (Port Hope, CA)
|
Assignee:
|
Milltronics Ltd. (Peterborough, CA)
|
Appl. No.:
|
914532 |
Filed:
|
July 17, 1992 |
Current U.S. Class: |
367/99; 367/903 |
Intern'l Class: |
G01S 015/00 |
Field of Search: |
367/903,3,99,135
381/55
|
References Cited
U.S. Patent Documents
4199246 | Apr., 1980 | Muggli | 367/101.
|
4255800 | Mar., 1981 | Patterson | 367/99.
|
4439846 | Mar., 1984 | Rodriguez | 367/99.
|
4596144 | Jun., 1986 | Panton et al. | 73/290.
|
4831565 | May., 1989 | Woodward | 367/99.
|
4850226 | Jul., 1989 | Allen, Jr. et al. | 367/903.
|
4890266 | Dec., 1989 | Woodward | 367/99.
|
4992998 | Feb., 1991 | Woodward | 367/99.
|
5079751 | Jan., 1992 | Woodward | 367/99.
|
5132940 | Jul., 1992 | Culbert | 367/135.
|
Foreign Patent Documents |
2151357A | Jul., 1985 | GB.
| |
Primary Examiner: Pihulic; Daniel T.
Attorney, Agent or Firm: Ridout & Maybee
Claims
I claim:
1. An acoustic pulse-echo ranging system comprising a transducer containing
a high impedance piezoelectric transducer element, a transmitter for
driving the transducer element, a receiver having an input for receiving
signals from the transducer element, and a transceiver interface for
placing said transmitter and said receiver continuously in connection with
the transducer element, wherein the transceiver interface comprises a
resistive network establishing a direct resistive connection between the
transducer element and the receiver, the network further including an
electronically controlled switching element for switching the network
between two states, namely a first state in which the transducer element
is coupled to the receiver through a first resistive element whose
resistance is low compared with the impedance both of the receiver and the
transducer such that the network neither substantially damps the
transducer element nor attenuates the input to the receiver, and a second
state in which the network substantially damps the transducer element and
attenuates the input to the receiver by establishing, through a second
resistive element, and additional resistive path directly in parallel with
the transducer element and having a resistance which is low compared with
the impedance of both the transducer and the receiver, and wherein the
system includes a controller for controlling the switching element to
select the state of the network according to the expected characteristics
of an echo to be received by the receiver.
2. The system of claim 1, wherein the second resistive element is of
substantially lower value than the first resistive element.
3. The system of claim 2, wherein oppositely polarized diodes are connected
in parallel with the second resistive element and the switching element,
and the transducer drives the transducer element at a level such that the
diodes conduct alternately during operation of the transmitter to place
the first resistive element in parallel with the transducer.
4. The system of claim 1, wherein the switching element is a switching
transistor.
5. The system of claim 3, wherein the transducer, the transmitter, the
receiver and the transceiver interface are located within a common
housing.
6. The system of claim 1, wherein the transmitter is connected to the
transducer through a transformer having a variable air gap.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to acoustic range finding systems of the type in
which an electro-acoustic transducer transmits a pulse or shot of acoustic
energy towards a surface whose distance is to be measured, and subsequent
signals received from the transducer are monitored to determine the
temporal location of an echo from that surface.
2. Review of the Art
In such a system, various compromises are necessary in accommodating the
characteristics of the transducer and its associated transmitter and
receiver. The reactances of the transducer and of circuits coupling the
transducers to the transmitter and receiver form a tuned circuit resonant
at a frequency at or near the operating frequency of the transducer, and
the quality factor or Q of this circuit has a profound effect upon the
amplitude of the acoustic energy generated, the sensitivity of the
transducer to return echoes, and the amplitude and rate of decay of the
"ringing" of the transducer following transmission of the shot. The higher
the Q, the greater the amplitude and sensitivity, but also the greater the
ringing, which may severely affect the ability of the system to detect
close-in echoes since these may either be swamped by the ringing, or
rendered undetectable through saturation of the receiver caused by high
amplitude ringing. In actual systems such as those described in commonly
assigned prior U.S. Pat. Nos. 4,831,565; 4,890,266; 4,992,998 and
5,079,751, the input of the receiver is protected against high amplitude
signals from the transmitter by placing opposite polarity pairs of
limiting diodes at the input of the receiver, in series with a limiting
resistor or resistors, such that during operation of the transmitter the
diodes conduct. The limiting resistor acts both to protect the receiver,
and to prevent too large a proportion of the transmitter energy being
dissipated in the diodes. The resistor or resistors also apply some
limited damping (i.e. reducing the Q) to the transducer circuit. During
reception of low amplitude echo signals, the diodes cease to conduct, thus
placing the resistor or resistors in series with a higher impedance or
impedances presented by the receiver input and somewhat reducing the
damping so as to provide improved sensitivity and noise immunity during
reception. Whilst this arrangement works well, it still requires design
compromises. The ringing characteristics of the transducer circuit are
difficult to control closely due to varying characteristics of individual
transducers, the reactance and resistance of varying lengths of
transmission line connecting them to the transmitter, and imperfections of
the impedance matching transformer usually located between the active
element of the transducer and the transmission line. These factors mean
that the limiting resistor or resistors can exercise only a small degree
of control over damping of the transducer. Sufficient damping must be
applied to ensure that clipping by the diodes will cease and the receiver
will be unsaturated within a time, following the start of a shot,
corresponding to the minimum range to be measured. Whilst various
techniques, such as shortened shots, may be used to provide a short
minimum range, the receiver requires a very large dynamic range to
accommodate the extreme differences in signal amplitude existing between
short range and distant echoes. This in turn has tended to require the use
of arrangements such as those disclosed in U.S. Pat. No. 4,596,144 (Panton
et al.), using multiple logarithmic amplifiers, to provide the necessary
dynamic range. An alternative approach has been to place an electronically
switchable attenuator at the input to the receiver which is switched in
for short range shots only in order to reduce the dynamic range required
in the receiver. Such an arrangement is used in the MULTIRANGER
(trade-mark) system of the assignee of this application.
Yet another approach has been to utilize variable gain control of the
receiver; see for example U.S. Pat. No. 4,439,846 (Rodriguez) which
represents a development of the Muggli patent discussed below.
Satisfactory wide range gain control can present considerable
implementation problems and leaves the necessity for the receiver to cope
with a very wide range of signal amplitudes at its input.
U.S. Pat. No. 4,199,246 (Muggli), issued Apr. 22, 1980, describes an
ultrasonic ranging system in which the transmitter is driven by a voltage
controlled oscillator, such that the frequency transmitted by the
transducer is changed in a predetermined manner over a substantial range
during the course of the transmitted pulse. The bandwidth of the receiver
is varied, again according to a preset pattern, during a subsequent period
by changing the receiving Q so that the receiver bandwidth is narrowed
with the passage of time following the pulse, the passband of the receiver
being centred upon the lowest frequency transmitted. By configuring the
transmitted pulse so that a short initial portion is transmitted at a
relatively high frequency, which is then reduced in one or more steps to a
relatively low frequency, and configuring the receiver so that its initial
bandwidth is wide enough to pass the highest as well as the lowest
frequency, short range echoes of the high frequency pulse components may
be detected, but at longer ranges, reception of the low frequency
component and exclusion of noise is optimized, by decreasing the bandwidth
and thus improving the quality factor (Q) of the receiver. The variable Q
circuit used in the receiver only however affects the receiver circuits,
and does not vary the damping applied to the transducer itself, whose
characteristics remain unmodified. The dynamic range of the signals
appearing at the receiver input is thus unmodified.
The Muggli system is subject to two constraints which limit its
applicability. The transducer itself must be capable of operation over a
wide range of frequencies, and the noise immunity of the system at short
ranges is very poor because of the wide bandwidth of the receiver at those
frequencies. Neither of these limitations need be serious in the camera
control applications for which the Muggli system is clearly primarily
designed, involving as they do low power transducers, comparatively short
ranges, and environments which are comparatively noise-free at the
frequencies of interest; they are however highly significant in typical
industrial applications for which suitable transducers operating over a
wide frequency range are not generally available. Instead it has been
necessary to select a suitable transducer, and to provide a
transmitter/receiver system whose frequency characteristics and output
voltage are matched to the transducer.
U.S Pat. No. 5,079,751 discloses a control unit for connection to at least
one electroacoustic transducer to form an acoustic ranging system,
comprising a transmitter for generating shots of alternating current
electric energy for application to each said transducer to generate
acoustic energy, a tuned receiver for receiving and amplifying alternating
current generated by each said transducer responsive to the receipt of
acoustic energy, means for digitizing output from said receiver, and a
control computer controlling said transmitter to time said shots and for
processing said digitized receiver output to recognize therein features
indicative of a primary echo from a target being ranged, said unit further
including first electronically controlled means for determining an
operating frequency of said transmitter, and second electronically
controlled means for causing the tuning of said receiver to track the
operating frequency of said transmitter, and said control computer further
controlling said first electronically controlled means to determine the
frequency of the transmitter during each shot responsive to data relative
to characteristics of each said transducer.
As well as the frequency related control mentioned above, the patent also
discloses how a main loading resistor connected in parallel with the
transmitter can be associated with an additional resistor which is
switched into parallel with the main loading resistor to adjust somewhat
the damping of the transducer and thus its Q or quality factor. The
optimum amount of damping may of course vary according to the transducer
utilized, hence my incorporation of means to modify the damping in the
unit which forms the subject of the above application, according to the
characteristics of the transducer utilized during each shot.
In practice the additional resistor has, as already discussed, only a
limited effect on the ringing characteristics of a transducer, since a
typical transducer in a multitransducer array such as is contemplated in
U.S. Pat. No. 5,079,751 will necessarily be in most instances remote from
the transceiver. Transducers suitable for such application incorporate a
built-in transformer so that the connecting cable to the transducers may
operate at relatively low impedance, and the combination of the
imperfections of the built-in transformer and the reactance of the cables
will mean that only limited damping can be applied by the additional
resistor even if its value is so low as to severely limit the transmitter
output. The arrangement is thus only suitable for achieving such minor
adjustments of ringing characteristics as are necessary to suit different
transducers.
With the continuing decrease in the cost of implementing even quite
sophisticated electronic digital signal processing, it becomes
increasingly practical to carry out echo processing at a location adjacent
the transducer, the resulting range information being monitored at a
remote location. It has been known for some time to provide local
preprocessing of echo data adjacent the transducer, for example from
published U.K. Patent Application 2151357A (Endress & Hauser GmbH & Co.),
in order to facilitate transmission of the data to a remote processing
unit, and more recently systems have appeared in which the data processing
also has been carried out adjacent the transducer.
SUMMARY OF THE INVENTION
I have now found that, where signal processing is carried out adjacent the
transducer, it is possible to use a technique superficially similar (in
that it relies upon switching in an additional resistor in a resistive
network) to that disclosed in U.S. Pat. No. 5,079,751, but in systems
designed for use with one particular transducer type in order to optimize
performance at different ranges, by employing a circuit at the receiver
input which acts at the same time both to provide variable damping of the
transducer and variable attenuation of the receiver input.
I modify the receiver input circuit already described above, which, in its
existing form applies additional damping to the transducer circuit only
when the signal level is above the clipping level of the diodes during and
immediately following the transmission of a shot, by selectively causing a
resistive load to appear directly in parallel with the piezoelectric
transducer element during some circumstances of operation of the receiver.
In a preferred arrangement such a resistance forms a potential divider in
conjunction with the limiting resistor, and thus attenuates the input to
the receiver in a defined proportion. At the same time it appears, when in
circuit, in series with the limiting resistor to apply substantial damping
to the transducer circuit during reception, thus increasing the rate of
decay of ringing and decreasing sensitivity of the transducer. The
resistive load is applied only when detecting short range echoes, or when
exceptionally high received signal amplitudes are present which would
otherwise saturate the receiver, both conditions which are readily
detected. A large measure of signal attenuation and damping of ringing can
be applied where the value of the resistive loading is much less both than
that of the limiting resistor and that of the input impedance of the
receiver. This is acceptable and indeed desirable when received signal
amplitudes are large, and the dynamic range which must be handled by the
amplifier is much reduced, thus enabling the latter to be simplified. It
should be understood that, for the invention to operate effectively, the
resistive loading must appear directly in parallel with the high impedance
piezoelectric element itself, without the imposition of a matching
transformer and a possibly lengthy transmission line as occurs in prior
art arrangements.
SHORT DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic diagram of an interface between a
transducer and acoustic transceiver showing a prior art arrangement; and
FIG. 2 is a simplified schematic diagram of an interface between a
transducer and an acoustic transceiver in accordance with the invention.
Referring to FIG. 1, a transmitter 2 is conventional, comprising a source
of direct current schematized by terminals B+ and G, a chopper circuit 4,
typically formed by a VFET (vertical field effect transistor) driven by a
bipolar transistor fed with digital control signals from a control
computer 6, which causes a burst or "shot" of acoustic energy to be
generated by turning the chopper on and off through a defined number of
cycles for defined periods at a defined repetition frequency, according to
the length of shot required and the resonant frequency of a piezoelectric
acoustic transducer 8. The chopper 4 is in series with the primary of a
double wound transformer 10, whose secondary winding is connected to a
transducer 8 by a transmission line 11, which may be a coaxial cable or
twisted pair of any required length up to several hundred meters. In order
to match this transmission line, and reduce the effect of the reactance of
the line which will vary greatly according to its length, the transducer 8
includes, as well as piezoelectric element 9, a matching transformer 7
which steps down the characteristically high impedance of the
piezoelectric element 9 to a much lower impedance, typically 400.OMEGA..
At its transceiver end the transmission line 11 is terminated by a
transceiver interface circuit, including a capacitor 5, which is selected
so as to swamp the capacitance of the line and very approximately resonate
with the inductive elements of the circuit at the frequency of the
transducer, and a resistance network, comprising resistors 14, 15 and 30,
the latter in parallel with back-to-back diodes 16 and 18. The values of
resistors 14 and 15 are comparatively high (typically 2000 ohms) compared
with the input impedance of the transducer (400.OMEGA.). The purpose of
the resistor 15 is to improve the matching of the transmitter to the
transmission line, and to ensure some loading of the transmitter to
protect the chopper in the event of an open transducer circuit. As
described in U.S. Pat. No. 5,079,751, such a resistor may be switched in
on of circuit to help adjust the matching of a particular transducer to
the transmitter. During transmission, the resistor 14 also appears in
parallel with resistor 15, but like resistor 13 it is of comparatively
high value (2000 ohms) compared with the input impedance of the transducer
so that it has comparatively little effect upon the damping of the latter.
During a transmit pulse or high amplitude ringing of the transducer, the
diodes 16 and 18 will conduct, placing resistor 14 in parallel with
resistor 15, and the same will occur when a switching transistor 32 is
turned on, placing a low value (typically 25 ohms) resistor 30 in
parallel with the diodes. In this latter case, it is more significant the
resistor 30 is placed in parallel with the much higher impedance input of
a receiver preamplifier 22, and in series with the resistor 30, thus
forming a potential divider which acts as an input attenuator to the
receiver. This transistor 32 controlling this attenuator is turned on when
measuring short ranges, thus reducing the dynamic range needed by the
receiver, but there will be little effect upon the damping of the
transmitter. It will be noted that under all circumstances, the transducer
element itself is isolated from both the transmitter and receiver by the
matching transformer 7 and the transmission line 11, which between them
act to isolate the transducer element to a large extent from the effect of
circuit characteristics at the far end of the transmission line: indeed
the use of a low impedance connection to the transducer is intended to
have just that effect.
Referring now to FIG. 2, in which like reference numerals are used to
denote like parts, it will be noted that both the transmitter, and the
transceiver interface circuit, are connected directly to the transducer
element, the transformer 7 and the transmission line being eliminated.
This becomes possible when the transceiver is implemented integrally with
the transducer.
Damping is applied to the transducer 8 when the transmitter is active by
the parallel connected resistor 14 connected in parallel with the
transducer via the limiting circuit formed by the back-to-back connected
diodes 16 and 18. One or other of these diodes will conduct whenever the
potential across them exceeds the forward conduction threshold of the
diodes, typically less that 1 volt. Since the peak-to-peak potential
applied to the transducer by the secondary of transformer 10, when the
transmitter is active, is of the order of hundreds of volts, the diodes
have a negligible effect on the damping applied by the resistor 14 during
operation of the transmitter. The inductance of the secondary winding of
the transformer is preferably selected to form a tuned circuit with the
capacitance of the transformer, to which end a controlled air gap 12 may
be introduced into the core of the transformer, both to control its
inductance and prevent saturation, and to help provide consistent
performance from the transformer/transducer combination.
Since there is no matching transformer between the resistor 14 and the
transducer element 8, the resistor has a substantial damping effect upon
the relatively high impedance transducer. Even though the resistor 14 may
be of higher value (for example 5000.OMEGA.) than in the arrangement of
FIG. 1, it will be of low value relative to the impedance of the
transducer element. This is in contrast to the arrangement of FIG. 1,
where the value of resistor 14 was much higher than the transformed
impedance (400 ohms), of the transducer. The preamplifier 22 has a high
input impedance at the transducer frequency, but the diodes 16 and 18,
which serve to limit the potential appearing at the input to the receiver
during transmission of a shot, and to determine a saturation level at the
input of the receiver since signal excursions beyond the forward
conduction thresholds of the diodes will be clipped, also complete a
current path through the resistor 14 during transmission of a shot.
The fixed gain preamplifier 22, which may be provided with some bandpass
characteristics, is followed in the receiver 24 by an active filter tuned
so as to pass components of the received signal at or close to the
transducer frequency, which in turn is followed by a logarithmic amplifier
and precision rectifier to produce a signal proportional, on a logarithmic
scale, to echo amplitude. An echo processing system (implemented by a
programmed microcontroller)) then digitizes and processes the echo signal
so as to recognize a true echo, using techniques similar to those
described for example in my U.S. Pat. Nos. 4,831,565, 4,890,266 and
4,992,998.
At the input to the receiver, and in parallel with the diodes 16 and 18, is
a further resistor 30 of lower resistance (typically 240 ohms) than the
resistor 14, in series with the collector and emitter of a bipolar
switching transistor 32, the base of which receives a signal from the
microcontroller which determines whether or not the transistor is turned
on. When the transistor is turned off, the resistor 30 has no effect.
During receipt of an echo signal following transmission of a shot, and
provided that the amplitude of the received signal is below the saturation
level of the receiver as set by the clipping diodes 16 and 18, the
received signal is applied directly to the preamplifier 22. The resistor
14 will be in series with the relatively high input impedance of the
amplifier 22, and will have little effect on the amplitude of the signal.
The tuned circuit formed by the transducer 8 and the transformer 10 will
see only a high impedance formed by the resistor 14 and the input
impedance of the amplifier in series, and therefore little damping will be
applied to the transducer, thus maximizing its sensitivity to received
echoes. When the transistor 32 is turned on, on the other hand, the
resistors 14 and 30 form a potential divider at the input of the amplifier
22, and since the resistor 30 has a much smaller value (typically 240
ohms) than the resistor 14 (typically 5000 ohms), much the same degree of
damping is applied to the transducer circuit as occurs during
transmission. Thus the input to the receiver is not only attenuated to
about 5% of its damped value, but the transducer is also damped to the
same degree as during transmission.
Damping of the transducer, by switching on the transistor 32, is applied
under two circumstances. When transmitting "short" shots to detect short
range echoes, as described further in my U.S. Pat. No. 4,831,565, the
expected return signals will be of high amplitude, and attenuation is
therefore applied. When transmitting "long" shots to detect longer range
echoes, damping will also be applied if the computer detects signal levels
within the range being examined which are high enough to saturate the
receiver. A substantial advantage of applying damping in this manner is
not only that it greatly extends the effective dynamic range of the
logarithmic amplifier 26, so that only one logarithmic amplifier need be
used, but also that the ringdown time of the transducer can be greatly
reduced by effective damping of the transducer, so that the minimum
effective range of the system is substantially reduced.
Whilst only a single stage of damping has been described, several stages of
damping could be applied by providing two or more individually controlled
sets of resistors 30 and transistors 32, or replacing this group of
components with a circuit providing a continuously controllable
resistance.
In a preferred physical arrangement, the transmitter, receiver and
transducer are integrated in a single housing 34. Since the
characteristics of transducers are notoriously difficult to control
accurately during manufacture, and substantial but unpredictable
reactances can be introduced by connecting wiring if the transducer is
located remotely from the transmitter and receiver, such an arrangement
provides the potential of adjusting the combination of transformer and
transducer as a unit during manufacture to provide a desired response and
correct deviations in the characteristics of individual transducers. The
inductance of the transformer can if required be adjusted by utilizing a
transformer core having an air gap, adjustable for example by rotation of
a screw threaded slug forming part of the core, or the reactance of the
circuit could be adjusted using an adjustable capacitance.
Top